1. Field of the invention
The present invention relates to the locating of faults in power distribution systems and, more particularly, to a method and apparatus for pinpointing the location of a fault occurring in an underground residential distribution (URD) system by real-time analysis of the fault signal.
2. Description of the Background
It is well-known that faults occurring along a power distribution system can immediately be detected by monitoring a fault signal which propagates as a consequence of the fault. Both phase-to-phase and phase-to-ground faults produce a fault signal in the form of a step-function which reflects throughout the system. Several conventional devices are currently available for detecting such fault signals. However, once the fault signal has been detected it is of equal importance to quickly repair the fault. For this, the fault must be promptly located. Prior art cable fault locators commonly make an ex post facto determination of the fault location. For instance, after a fault has already occurred, the distribution system is taken off-line. Fault-tracing signals are then injected at different points along the distribution circuit, and the fault location is traced based on the circuit response. This process is unduly time consuming, and often results in a lengthy power outage until the fault is located and repaired.
A few devices exist which assess the location of the fault based upon the characteristics of the transient fault signal itself. This is a considerable improvement because the fault can be located immediately upon its occurrence. The above-described lengthy troubleshooting process is avoided, and outage time is minimized.
For example, U.S. Pat. No. 3,710,239 to Nakamura discloses a fault monitoring device which operates with sensors placed at both ends of an isolated section of transmission line. One sensor detects the leading edge of a fault signal and thereupon activates a counter. When the fault signal reaches the second monitor it activates a transmitter which signals the counter to stop. As long as the fault occurs between the sensors, the contents of the counter will be proportional to the fault location. Hence, the contents of the counter can be used to localize the fault.
U.S. Pat. No. 4,857,854 to Matsushima discloses a digital fault locator which measures voltage and phase currents due to a fault signal. This device implements a digital filtering technique in calculating the fault location based upon the fault signal.
U.S. Pat. No. 4,797,805 to Nimmersjo uses a travelling wave model to describe the voltage distribution along the transmission line. The model results are compared to actual measurements of the voltage at both ends of the transmission line. Given the length of the transmission line, the distance to the fault can be calculated.
Although the above-described patents provide an instantaneous indication of the fault location, they all disclose devices which require a plurality of sensors positioned along a transmission line, and the sensors must monitor both line current and voltage. Moreover, the technique used in calculating the fault location is exceedingly complex, which necessitates costly implementing circuitry.
U.S. Pat. No. 4,500,834 to Ko et al. suggest a less complicated and more economical fault locator for an overhead transmission system. Ko et al use a single monitor placed along the transmission line. The monitor is placed in an isolated zone of the transmission line which is protected by relays. The monitor detects the leading edge of a fault signal and monitors the amount of time required for the signal to reflect from the monitor to the end of the isolated zone and back to the monitor. The elapsed time is then compared to a preset time to determine whether the fault occurred within the isolated zone. If so, the device triggers the appropriate relay. Unfortunately, the applications of the Ko et al device are limited. This is because the device was designed only for use with an overhead transmission system. Consequently, a number of design constraints arise. The Ko et al device must be used in a relay-protected zone, and it merely indicates when a fault occurs within the zone. It does not disclose a means for accurately pinpointing the location of the fault. Moreover, the device must ascertain the direction in which a fault signal is propagating. For this, dual sensors are required to sense both current and voltage, and a complex directional fault detection unit is required.